Controlling inrush current from a power supply to a load

1. Apparatus for controlling inrush current from a power supply to a load, the apparatus comprising:

a first and a second power supply input and a first and a second load output;

a semiconductor device coupled between the first power supply input and the first load output;

a control circuit having a control circuit input coupled to the first load output and a control circuit output coupled to a conductance control node of the semiconductor device,

wherein the apparatus is configured such that when a load is connected across the first load output and the second load output, the control circuit provides a control signal to the conductance control node to thereby progressively vary the conductance of the semiconductor device from a low conductance state to a high conductance state in response to a load signal being provided to the control circuit input indicating that a load current flowing through the semiconductor device has exceeded a pre-determined threshold,

wherein the load signal is indicative of a voltage at the first load output and the control circuit is arranged to provide the control signal in response to determining whether the voltage at the first load output is less than a threshold voltage, and

wherein the control circuit comprises an op amp having a voltage output coupled to a control resistor and a control capacitor which is also coupled to the conductance control node of the semiconductor device, the op amp having inputs coupled to the control circuit input and the threshold voltage.

2. Apparatus as claimed in claim 1 , wherein the conductance of the semiconductor device is progressively varied from the low conductance state to the high conductance state over a predetermined duration dependent on a capacitance of the load.

3. Apparatus as claimed in claim 2 , wherein the predetermined duration is at least the duration required to allow the capacitance of the load to charge to a steady state voltage between the first and second load outputs of the apparatus.

4. Apparatus as claimed in claim 1 , wherein the semiconductor device comprises a p-channel MOSFET.

5. Apparatus as claimed in claim 1 , further comprising a voltage divider coupled between the first and second power supply inputs of the apparatus in order to generate the threshold voltage.

6. A power supply system comprising:

a power supply coupled to an apparatus for controlling inrush current from the power supply to a load, the apparatus comprising:

a first and a second power supply input coupled to the power supply, and a first and a second load output;

a semiconductor device coupled between the first power supply input and the first load output;

a control circuit having a control circuit input coupled to the first load output and a control circuit output coupled to a conductance control node of the semiconductor device,

wherein the apparatus is configured such that when a load is connected across the first load output and the second load output, the control circuit provides a control signal to the conductance control node to thereby progressively vary the conductance of the semiconductor device from a low conductance state to a high conductance state in response to a load signal being provided to the control circuit input indicating that a load current flowing through the semiconductor device has exceeded a pre-determined threshold,

wherein the load signal is indicative of a voltage at the first load output and the control circuit is arranged to provide the control signal in response to determining whether the voltage at the first load output is less than a threshold voltage, and

wherein the control circuit comprises an op amp having a voltage output coupled to a control resistor and a control capacitor which is also coupled to the conductance control node of the semiconductor device, the op amp having inputs coupled to the control circuit input and the threshold voltage.

7. A method of controlling inrush current from a power supply to a load, the method comprising:

progressively varying the conductance of a semiconductor device coupled between the power supply and the load from a low conductance state to a high conductance state in response to detecting connection of the load to the semiconductor device by determining that a load current flowing through the semiconductor device has exceeded a pre-determined threshold by determining that a voltage at a load output node coupled between the semiconductor device and the load exceeds a threshold voltage, the varying of the semiconductor device employing voltage control and having a predetermined duration dependent on a capacitance of the load,

wherein the duration RC is:

RC

≥

τ

=

2

⁢

C

L

⁢

V

S

⁡

(

V

S

-

V

T

)

⁢

K

1

/

2

I

T

3

/

2

where C L , is a capacitance value of a capacitive part of the load, K and V T (threshold voltage) arc constants dependent on the semiconductor device, V s is a voltage at a source of the semiconductor device, and I T is a permissible inrush current.

8. A method of controlling inrush current as claimed in claim 7 , wherein determining that a load current flowing through the semiconductor device has exceeded a pre-determined threshold comprises detecting a predetermined semiconductor device voltage across the semiconductor device.

9. A method of controlling inrush current as claimed in claim 7 , wherein determining that a load current flowing through the semiconductor device has exceeded a pre-determined threshold comprises detecting a predetermined load voltage across the load.

10. A method of controlling inrush current as claimed in claim 7 , wherein the predetermined duration is at least the duration required to allow the capacitive load to charge to a steady state voltage.

11. Apparatus of claim 1 , wherein an RC time constant of the control resistor and control capacitor is:

RC

≥

τ

=

2

⁢

C

L

⁢

V

S

⁡

(

V

S

-

V

T

)

⁢

K

1

/

2

I

T

3

/

2

where C L is a capacitance value of a capacitive part of the load, K and V T (threshold voltage) are constants dependent on the semiconductor device, V s is a voltage at a source of the semiconductor device, and I T is a permissible inrush current.

12. Apparatus of claim 11 , wherein the RC time constant is at least several times τ.

13. The power supply system of claim 6 , wherein an RC time constant of the control resistor and control capacitor is:

RC

≥

τ

=

2

⁢

C

L

⁢

V

S

⁡

(

V

S

-

V

T

)

⁢

K

1

/

2

I

T

3

/

2

where C L is a capacitance value of a capacitive part of the load, K and V T (threshold voltage) are constants dependent on the semiconductor device, V s is a voltage at a source of the semiconductor device, and I T is a permissible inrush current.

14. The power supply system of claim 13 , wherein the RC time constant is at least several times τ.

15. The method of claim 7 , wherein the duration RC is at least several times τ.